1. Field of the Invention
The invention relates generally to encoding data, and more particularly to data encoding methods and apparatus for a partial-response maximum-likelihood (PRML) data channel in a direct access storage device (DASD).
2. Description of the Prior Art
Computers often include auxiliary memory storage units having media on which data can be written and from which data can be read for later use. Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Data is recorded in concentric, radially spaced data information tracks arrayed on the surfaces of the disks. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. Partial-response signaling with maximum-likelihood sequence detection techniques are known for digital data communication and recording applications. Achievement of high data density and high data rates has resulted in the use of a PRML channel for writing and reading digital data on the disks.
Uncoded binary data is not suitable for use in PRML data channels because unconstrained customer data may contain long spans of null signal or adjacent zeroes which provides no timing or gain information to the channel and prevent proper timing and gain tracking to the read back signal waveform. Rate 8/9 modulation codes are known for use with PRML channels to assure a minimum correction rate for the PRML timing and gain control loops.
U.S. Pat. No. 4,786,890 discloses a class-IV PRML channel using a run-length limited (RLL) rate 8/9 code. The disclosed class-IV partial response channel polynomial equals (1-D.sup.2), where D is a delay operator and D.sup.2 is a delay of 2 bit times and the channel response output waveform is described by taking the input waveform and subtracting from it the same waveform delayed by a 2 bit interval. A (d=0,k=3/k1=5) PRML modulation code is utilized to encode 8 bit binary data into codewords comprised of 9 bit code sequences, the maximum number k of consecutive zeroes allowed within a code sequence is 3 and the maximum number k1 of consecutive zeroes in the all-even or all-odd subsequences is 5. A minimum number d of consecutive zeroes greater than zero is not needed because in PRML channels compensation for intersymbol interferences (ISI) is inherent in the ML detector.
U.S. Pat. No. 4,707,681 discloses rate 8/9 RLL block codes having (0,4/4) and (0, 3/6) constraints.
Disadvantages of known codes relate to timing and gain control and susceptibility to misequalization effects in PRML channels for at least some of the codewords in the codes.